use strict;
use warnings;

my $string = '99999999999999999999999999';
my $string2 = '9-999-999-999-999-999-999-999-9999'; # see Note 1 below

if ( $string =~ /^(\d)(\1|-)+$/ ) {
   print "\$string might be a phone number: $string \n";
} else {
   print "$string is NOK \n";
}

if ( $string2 =~ /^(\d)(\1|-)+$/ ) {
   print "\$string2 might be a phone number: $string2 \n";
} else {
   print "$string2 is NOK \n";
}

print "stiller's snippet: \n";

my $num = 999999999;
print sprintf("%f\n",$num);
print sprintf("%d\n",$num);

##</code><code>##

$string might be a phone number: 99999999999999999999999999       # see Note 2
$string2 might be a phone number: 9-999-999-999-999-999-999-999-9999 # also see Note 2
stiller's snippet:
999999999.000000  # a float, as stiller noted, this address's OP's problem,
999999999         # whereas, Perl makes it easy to use this as a string